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IC 


9058 



Bureau of Mines Information Circular/1986 



Anchorage Capacities in Thick Coal Roofs 



By Stephen C. Tadolini 




UNITED STATES DEPARTMENT OF THE INTERIOR 



Information Circular 9058 



Anchorage Capacities in Thick Coal Roofs 



By Stephen C. Tadolini 



UNITED STATES DEPARTMENT OF THE INTERIOR 

Donald Paul Hodel, Secretary 

BUREAU OF MINES 
Robert C. Horton, Director 




Trims 
•ft 

no. 9dsx 



Library of Congress Cataloging in Publication Data: 



Tadolini, Stephen C 

Anchorage capacities in thick coal roofs. 

(Information circular / United States Department of the Interior, 
Bureau of Mines ; 9058) 

Bibliography: p. 11-12. 

Supt. of Docs, no.: I 28.27: 9058. 

1. Coal mines and mining— Safety measures. 2. Mine roof bolting. 
I. Title. II. Series: Information circular (United States. Bureau of 
Mines) ; 9058. 



TN295.U4 622s [622'. 334] 85-600220 



tf- 

*3- 

J) CONTENTS 

"^ Page 

Abstract 1 

Introduction 1 

Acknowledgments 2 

Pull test: A method for determining the strength of a rock bolt anchor 2 

Characteristics of roof bolting systems 2 

Atlas Copco Swellex system 2 

Combination system 3 

Mechanical system 4 

Fully resin-grouted system 4 

Strain gauge system 5 

Pull test results 5 

Atlas Copco Swellex bolts 6 

Combination bolts 6 

Mechanical expansion anchor bolts 7 

Resin-grouted bolts 7 

Strain-gauge bolts 7 

Representation of pull test by finite element model 9 

Results of finite element analysis 10 

Conclusions 11 

References 11 

Appendix. — Torque-tension ratio tests.. 13 

ILLUSTRATIONS 

1. Tested roof bolts V 3 

2. Atlas Copco Swellex bolt manufacturing sequence 4 

3 . Atlas Copco Swellex bolt expansion sequence 4 

4. Strain-gauge bolts 5 

5. Pull test apparatus 6 

6. Swellex bolt pull test results 6 

7. Combination bolt pull test results 7 

8. Mechanical expansion anchor bolt pull test results 7 

9. Resin-grouted bolt pull test results 8 

10. Strain-gauge locations and resin columns in strain-gauge bolts 8 

11. Load-strain curve for strain-gauge bolt with 24-in resin column 9 

12. Load-strain curve for strain-gauge bolt with 36-in resin column 9 

13. Finite element model 10 

14. Typical plot of percent of applied load versus bolt distance 10 

TABLE 

1. Physical properties of materials used in finite element model 9 



<4» 






o 
—ft 





UNIT OF MEASURE ABBREVIATIONS USED IN THIS REPORT 


ft 


foot lb/in 3 pound per cubic inch 


in 


inch yin microinch 


lb 


pound psi pound per square inch 


lbf-ft 


pound (force) foot 



ANCHORAGE CAPACITIES IN THICK COAL ROOFS 

By Stephen C. Tadolini 



ABSTRACT 

The anchorage capacity of four types of bolting systems was investi- 
gated in two mines with thick coal roofs. In conjunction with the in- 
mine tests, a simple axisymmetric finite element computer program was 
developed. Both the field and computer results indicated that thick 
coal roofs can be effectively supported entirely by the coal member. 

INTRODUCTION 

Many coal mines in the Western United States are located in thick coal 
seams that often leave the mining operators several feet of coal to form 
the mine roof. Information is needed concerning effective roof support 
in such mines. In this investigation of bolt anchorage capacities in 
thick coal roof, the following parameters were investigated: 

1. Type of anchorage — mechanical or chemical, either fully or par- 
tially bonded, using resin or cement as the bonding agent. 

2. Length of anchor — short (4 ft or less), medium (4 to 6 ft), or 
long bolts (6 ft or more). 

3. Diameter of anchor — small (less than 0.5 in), medium (from 0.5 to 
1.0 in), or large (greater than 1.0 in). 

To establish basic support criteria for mines with thick top coal, 
various types of roof bolts were investigated in small-scale in-mine 
tests. The bolts were pull tested to determine the average amount of 
support capacity that could be expected. In conjunction with this field 
investigation, a simple axisymmetric finite element model was devel- 
oped to simulate pull tests. The model simulated a 3/4-in-diam bolt 
installed in a 1-in-diam hole in a thick coal roof. To ensure equilib- 
rium and compatibility, deformation lengths were predetermined using 
strain-gauge bolts. The information obtained from the pull tests was 
compared with data derived from the finite element model to provide a 
fundamental approach for designing an adequate support system for thick 
coal roofs. 



^ Mining engineer, Denver Research Center, Bureau of Mines, Denver, CO. 



ACKNOWLEDGMENTS 



The author expresses gratitude to the 
Consolidation Coal Co. and Valley Camp 
of Utah Inc. for their continued support 
of ground control research. Special 
thanks are extended to Steven Jaccaud, 
superintendent of Consolidated 1 s Emery 



No. 50 Mine, Emery, UT, and Virgil Lamb, 
superintendent of Valley Camp's Belina 
No. 1 Mine, Scofield, UT, who made avail- 
able personnel and equipment for this 
study. 



PULL TEST: A METHOD FOR DETERMINING THE STRENGTH OF A ROCK BOLT ANCHOR 



Pull tests were conducted in two mines. 
In the first mine, a total of 40 bolts 
of four different types were tested. The 
four types were — 

1. Atlas Copco Swellex, 2 a bolt that 
is expanded by high water pressure, 

2. Combination resin-grouted and me- 
chanical bolts, 

3. Mechanical expansion anchor bolts, 
and 

4. Resin-grouted bolts. 

The bolts were installed in a roof that 
consisted of 8 ft of coal as predeter- 
mined with a fiber-optic stratascope. 
Various torque and expansion pressures 
were used, except that these parameters 
did not apply to the resin-grouted bolt 
installations. In the second mine, only 
fully resin-grouted bolts were tested, 
again in a roof with thick top coal. 



Pull tests were performed on each bolt 
to determine the bolting effectiveness of 
each individual bolt type. 

The pull test is intended to measure 
the strength of a rock bolt anchorage 
system under field conditions. This is 
based on the premise that the best way 
to compare data obtained from anchorage 
tests is to analyze the load-deformation 
curve produced from plotting field infor- 
mation. However, information obtained 
from pull tests performed on full-column 
grouted bolts reveals very little about 
anchorage capacity. Tests on such bolts 
usually determine only the strength of 
the steel used to manufacture the bolt. 
This makes it imperative that shorter 
grout columns be investigated to deter- 
mine actual anchorage capacities. 



CHARACTERISTICS OF ROOF BOLTING SYSTEMS 



Roof bolts have become an essential 
means of support in all types of under- 
ground openings. Numerous types and con- 
figurations of bolts are presently uti- 
lized to support excavations under all 
types of conditions. 

Previous investigations have involved 
both the developmental and evaluation 
stages of virtually all roof support de- 
vices (l_-4). 3 In this investigation, 
pull tests were conducted on the four 

— _ 

^Reference to specific products does 
not imply endorsement by the Bureau of 
Mines . 

3 Underlined numbers in parentheses re- 
fer to items in the list of references 
preceding the appendix. 



types of bolts previously mentioned. 
These bolt types are illustrated in 
figure 1. Strain-gauge bolts were also 
pull tested. The bolt types tested are 
not the only types deemed to be accept- 
able; they were, however, the bolts being 
considered by mine operators in the area 
where the tests were conducted at the 
time of the investigation. 

All bolt characteristics discussed in 
the following sections are minimum labo- 
ratory values. In some cases, the char- 
acteristics vary from bolt to bolt. 

ATLAS COPCO SWELLEX SYSTEM 

The Swellex water-pressure expansion 
bolt is manufactured from steel pipes 



1 '■ ■ ■ 



*D 



FIGURE 1. - Tested roof bolts. A, Atlas Copco Swellex; B, combination; C, mechanical expansion 
anchor; D, resin grouted. 



with a 1.61-in OD and a 0.08-in wall 
thickness. The pipes are reshaped to the 
Swellex profile with a 1.00-in OD as 
shown in figure 2. The bolt ends are 
strengthened with short support sleeves 
and sealed with welds. A small hole 
in the outer end of the bolt allows a 
high-pressure pump to inject water into 
the bolt, which then expands in the bore- 
hole, as schematically shown in figure 3. 
The bolt starts to expand at 880 to 1,180 
psi. This expansion pressure causes the 
shell of the tube to deform plasti- 
cally around irregularities in the bore- 
hole wall, thus creating a strong mechan- 
ical bond between the bolt and the 
rock. 

The Swellex bolt also works in smooth 
and straight (diamond-drilled) boreholes. 
Anchoring is achieved as a result of the 
high water pressure being transmitted to 
the borehole wall and causing the bolt 
and rock to expand elastically a few hun- 
dredths of an inch. After installation, 
when the water pressure is released (and 
the water flows out of the injection hole 
in the outer end of the bolt) , rock re- 
laxation is prevented by the expanded 



bolt, resulting in a residual pressure 
between the bolt and the borehole wall. 
The configuration of the expanded bolt 
combined with its material properties 
creates a spring action. 

An additional support characteristic 
of the bolt is its ability to provide 
active support. As the bolt is expanded, 
a small reduction in length occurs in the 
lower portion of the bolt. This creates 
tension in the bolt, which holds the 
bearing plate firmly against the rock. 
However, the resultant pressures can 
fracture brittle rock. This fracturing 
can be prevented by sliding a short tube 
over the bolt to prevent expansion of the 
bolt near the collar of the hole (5). 

COMBINATION SYSTEM 

The combination bolt utilizes resin- 
grouted rods for anchorage and mechanical 
bolts for tensioning the bolting system. 
The combination bolts used in this study 
consisted of a 2-ft length of 7/8-in-diam 
rebar coupled with a 3-ft length of 5/8- 
in-diam grade 55 steel (12,400 lb yield 
and 19,200 lb tensile strength). The 




FIGURE 2. - Atlas Copco Swellex bolt manu- 
facturing sequence. 

physical distinction of the combination 
system is the length of the resin 
anchor — 24 in or greater. The anchor en- 
cases a threaded joint in which the 
mechanical bolt can be inserted. The 
resin-grouted portion of the combination 
system not only provides anchorage for 
the device, but also reinforces the 
strata like a fully grouted bolt. The 
mechanical portion of the system can be 
adjusted to exert a desired tension on 
the bolt. This force is traditionally 
measured in torque and can be varied to 
limit deformation or prohibit deformation 
entirely. 










ililiiiiliilii; 3 wmzM* 

FIGURE 3. - Atlas Copco Swellex bolt ex- 
pansion sequence. 

MECHANICAL SYSTEM 

Mechanical bolting systems utilize a 
split-block, wedge-type expansion shell 
to create anchorage and support loads 
generated through the tensioned portion 
of the bolt. The mechanical bolts used 
in this study were 5/8-in-diam grade 55 
steel (12,400 lb yield and 19,200 lb ten- 
sile strength) . 

The success of a mechanical system is a 
function of its anchorage capacity. Tra- 
ditionally, clamping loads have been re- 
stricted to one-half of the yield point 
or anchorage capacity of the material in 
order to preserve both the integrity of 
anchorages and the bolt itself. To 
ensure that proper torque levels were 
used in the thick top coal, torque- 
tension ratio tests were performed. (See 
appendix. ) 

FULLY RESIN-GROUTED SYSTEM 

Fully grouted bolts rely on the mechan- 
ical interlock of the resin and inter- 
stices within the confines of the bore- 
hole wall and along the surfaces of the 



reinforcing rod to bind roof strata to- 
gether. The bolts used in this study 
were comprised of 3/4-in-diam reinforc- 
ing steel rod, grade 40 (17,600 lb yield 
and 30,000 lb tensile strength) , grouted 
in a 1-in-diam hole. No lateral forces 
are generated with fully grouted bolts 
at the time of installation, but high 
levels of anchorage can be expected. The 
horizontal reinforcement characteristics 
of the bolt permit it to carry high 
sheer-resistance values, with moderate 
stiffness resistance (resistance to 
bending) . 

STRAIN-GAUGE SYSTEM 

Strain-gauge bolts (fig. 4) were con- 
strucetd to be pull tested in conjunction 



with the other bolting systems. The 
strain-gauge bolts used were 1-in-diam 
rolled rebar bolts, each with a hole ap- 
proximately 0.125 in in diameter along 
the axis at the centerline. Six sets of 
strain gauges were placed on 5-ft bolts 
at three levels. The three gauge levels, 
as measured from the top of the threaded 
end, were 6 in, 27 in, and 57 in, re- 
spectively. A total of 10 bolts were in- 
stalled with 2 to 3 ft of resin grout, 
enabling the strain gauges at the 27-in 
level to be monitored with and without 
resin grout. The bolts were pull tested 
to obtain the traditional load-displace- 
ment curve, as well as the strains mea- 
sured by the gauges at 1,000-lb incre- 
ments of load (6). 



PULL TEST RESULTS 



The bolts were installed in the test 
sites and pull tested approximately 24 h 
later. This ensured an adequate resin 
cure and permitted retorquing of the com- 
bination and mechanical bolts. To ensure 
proper anchorage, the diameters of the 
drilled holes were measured with a flexi- 
ble hole gauge. Every fifth hole was 
measured at 1-ft intervals, with a maxi- 
mum fluctuation of 1/16 in in diameter 
being deemed acceptable. The drill-hole 
lengths were also corrected to compensate 



for the 1-in pull collar that was needed 
at the head of each bolt. For example, a 
60-in bolt was placed in a hole drilled 
to a depth of 59 in. Because all the 
support systems tested require the same 
pull-ring, no corrections were necessary 
for the subsequent analysis. The bolts 
were pull tested in accordance with the 
methods recommended by the International 
Society for Rock Mechanics (7_) . The pull 
test apparatus used in the investigation 
is represented in figure 5. 




FIGURE 4. - Strain-gauge bolts. 



Mine roof 




Roof bolt 

and 
bearing plate 



Pulling collar 



Aluminum 
housing 



Hydraulic piston 
assembly 




Deformation gauge 



" To mine floor 
FIGURE 5. - Pull test apparatus. 

ATLAS COPCO SWELLEX BOLTS 

The Swellex bolts were expanded using 
two installation pressures — 3,087 and 
4,410 psi, respectively. The results 
varied, with the yield point for the 
bolts installed at 3,087 psi (fig. 6) 




o.i 



0.2 0.1 0.2 

DEFORMATION, in 



FIGURE 6. - Swellex bolt pull test results. 
5-ft bolts, 1.375-in-diam holes, at installation 
pressures shown. 

being reached at an average value of 9.5 
tons. The deformation at this value 
was <0.1 in. The bolts installed with 
4,410 psi of installation pressure 
yielded at an average load of approxi- 
mately 11 tons. 4 The deformation at this 
point was also <0.1 in. 

COMBINATION BOLTS 

The combination bolts were installed 
using two different torque loads. The 
results of the pull tests showed that 
when the bolt was installed with 200 lbf* 
ft of torque (fig. 7) , the average yield 
point was 10.5 tons with displacements of 
<0.1 in. The bolts installed with 150 
lbf»ft of torque failed completely or 
just met the testing criteria selected 
for this study (8 tons of load with no 
more than 0.2 in displacement). Because 
pull tests are primarily concerned with 
the vertical movements related to the 
anchor of the bolt, these results might 
appear to be unreasonable upon cursory 
examination. All of the combination 
bolts were installed with identical an- 
chors consisting of 2 ft of resin grout. 

4 In this report, "ton" indicates 2,000 
lbf. 




0.2 0.3 0.1 

DEFORMATION, in 



0.2 



FIGURE 7. - Combination bolt pull test re- 
sults. 5-ft bolts, 0.75indiam, with 24-in res- 
in columns, torques as shown. 

However, if the pretensioning of the 
grouted length is taken into considera- 
tion, it would be possible to experience 
a higher level of resistance for the same 
increment of load for both torque lev- 
els. The grouted portion is placed into 
a higher degree of tension by the in- 
creased amount of force applied at the 
coupling. This grouted portion would, 
therefore, show a higher yield strength 
and, at the same time, less deformation. 

MECHANICAL EXPANSION ANCHOR BOLTS 

Mechanical bolts support the roof by 
suspension, friction, and keying. The 
effectiveness of mechanical bolting sys- 
tems is determined by the quality of 
their anchorage in the rock. In all 
instances of bolt failure in the thick 
coal roof, the strength of the bolt was 
not approached, but the anchor failed 
(fig. 8). Even when complete failure did 
not occur, anchor slippage was observed. 
The basic cause of the anchorage problem 
was stress concentrations at the anchor- 
age point. These concentrations can be 
effectively dissipated by increasing the 
contact area of the anchor by increasing 
its length or diameter. The coal's low 
compressive strength prevented most of 
the keying anchorage. Failures occurred 
at an average of 4 tons, with failure 




0.2 0.3 
DEFORMATION, in 

FIGURE 8. - Mechanical expansion anchor 
bolt pull test results. 



0.5 



considered as >0.2 
displacement. 



in total anchorage 



RESIN-GROUTED BOLTS 

The resin-grouted bolts were installed 
using various grout lengths, starting at 
2 ft and then increasing in 1-ft incre- 
ments to full column length. The results 
indicated <0.2 in bolt yield, regard- 
less of column length, when the bolt 
was subjected to 8 tons of load. The 
bolts yielded at an average of 10.5 tons, 
and displacement never exceeded 0.20 in 
(fig. 9). The resin bolts achieved good 
anchorage even in weak rock. One possi- 
ble explanation could be that the grout 
bonded all the cracks where dilation 
was possible. This action produces a 
suspension effect, confining stretching 
to a short section of the bolt. 

STRAIN-GAUGE BOLTS 

Pull test results for the strain-gauge 
bolts were similar to those for the 
resin-grouted bolts. Small amounts of 
displacement occurred as a result of bolt 
elongation as opposed to anchor slippage. 
The 5-ft bolts were installed using 24- 
and 36-in-long resin columns. The gauge 
locations and resin columns are shown 
in figure 10. The setup enabled strains 




0.2 



0.3 



0.1 



0.2 



0:3 0.1 02 

DEFORMATION, in 



0.3 



0.1 



02 



0.3 



0.4 



FIGURE 9. - Resin-grouted bolt pull test results. 5-ft bolts, 0.75 in diam, resin column lengths as 
shown. 



• 24" resin column --36" resin column 



± 



£ 











<e 3 "-» «J 



24" 



27' 



■Sf — 6"- 



>*- 4 



A 



33"- 



57"- 



60 



FIGURE 10. - Strain-gauge locations and resin columns in strain-gauge bolts. 



to be recorded with varying amounts of 
grout. The strain gauges indicated dis- 
tinctive stress patterns at the three in- 
strumented levels. 

The first type of field test was under- 
taken to determine strain gauge responses 
along the length of the bolt. A 5-ft 
bolt was installed with 2 ft of resin 
grout. This placed the grout 3 in above 
the gauge located at the 27-in level. 
The results, shown in figure 11, indi- 
cate that the gauges at the bottom and 
middle positions behaved practically 
identically. Also, the strains measured 
by these two gauges corresponded with the 
calculated strains predicted by the fol- 
lowing equation: 



where 



P = EeA, 
P = load, lb, 



and 



E = Young's modulus, psi, 

e = strain, 10~ 6 in, 

A = area of bolt, in 2 (the effec- 
tive area for the strain 
gauge bolts in 0.55 in 2 ). 



The gauge at the 57-in level was encased 
by 21 in of grout. Its response was ap- 
proximately 6% of the strains measured 
in the unencapsulated gauges. This gauge 
response was highly influenced by end ef- 
fects. The results do illustrate, howev- 
er, how rapidly the loads dissipate when 
the grout column is encountered. 

The second type of field test analyzed 
bolts that were installed with 3 ft of 
resin grout. Again, the bottom gauge was 
unencapsulated to check the system re- 
sponse. The middle and top gauges were 




KEY 

Gauge position: 
a Top 
■ Middle 
• Bottom 



200 300 400 500 600 700 
STRAIN, uin 

FIGURE 11. - Load-strain curve for strain- 
gauge bolt with 24-in resin column. 

encapsulated in 9 in and 33 in of resin 
grout, respectively. The results, shown 
in figure 12, indicate that the bolt re- 
sponse, measured by the bottom gauge, is 
similar to the calculated response. The 



- t i i r—f — i 1 

4 ' / jT 


\ P- 


if) / /^ 
o / / 

< / / KEY 

3 , J _/ Gauge position: 


- 




/ / a Top 




j / f/ ■ Middle 






/ • Bottom 




'"■ / X 


- 


\\ ' ' ' ' ' 


i 



100 



200 



300 400 

STRAIN, uin 



500 



600 



700 



FIGURE 12. - Load-strain curve for strain- 
gauge bolt with 36-in resin column. 

response of the middle gauge (encapsu- 
lated by 9 in of grout) was approximately 
53% of the bottom gauge response. The 
top gauge, 33 in from the grout line, 
showed negligible amounts of strain. The 
test results illustrate that load trans- 
fer in the bolt system takes place almost 
totally within the bolt at the load lev- 
els tested. 



REPRESENTATION OF PULL TEST BY FINITE ELEMENT MODEL 



To obtain a better understanding of 
pull tests, field tests were used to ver- 
ify the validity of a simple finite 
element computer model that provides a 
fundamental approach for designing rock 
anchor support systems for thick coal 
roofs (8-9)» The model (fig. 13) simu- 
lates a 4-ft bolt with a 3/4-in diam, 
installed in a 1-in-diam hole. Stresses 
were applied to the end of the bolt in 
100-psi increments, beginning with 100 
psi and ending with 1,500 psi. A test 
run was also made at 5,000 psi on the 
bolt head. The model was three-dimen- 
sional axisymmetric with rotation about 
the centerline of the bolt. The prop- 
erties of the various material compo- 
nents used in the analysis are listed in 
table 1. 



TABLE 1. - Physical properties of 
materials used in finite element 
model 



Property 


Coal 


Grout 


Steel 


Young's modulus (E) 








10 6 psi.. 


0.50 


0.30 


30 


Poisson's ratio (v). 


0.26 


0.25 


0.30 




0.0486 


0.0804 


0.2604 


Compressive strength 








(C) psi. . 


1,000* 


5,000 


20,000 


Angle of internal 




friction (<j>)....°.. 


46 


35 





Tensile strength (t) 








psi. . 





5,000 


20,000 



l klso tested at 1,500, 2,000, 
5,000, 7,500, and 10,000 psi. 



3,500, 



10 



RESULTS OF FINITE ELEMENT ANALYSIS 



The materials in the pull test model 
were assumed to be homogeneous and 
isotropic. Two types of cases were 
analyzed: 

1. Hold all material components con- 
stant and vary the amount of load placed 
on the end of the bolt. 

2. Keep the load on the end of the 
bolt constant and vary the compressive 



<L 





































\ 


' 


\ 


1 




J 


63 






















60 - 






























































/ 


























55 - f= 


^* 
















\ 


*S 








/ 






















1 












* 


RH 






















3U It 


















=fe 










































* 


ID ■" 








































* 
























/in I. 










'tU ■ 








^ 










































% 


■*R I- 






















OD ■ 


















^ 


































■*n 








$ 


ou 








































k 
























?*\ 










C.D 








^ 










































=fe 


on 






















cSJ r 


















* 


































ir 








*• 










































^ 
























in 










IU 








* 










































* 


s . 








































* 


































• ■ 










\ 


* 



10 



20 



25 



FIGURE 13. - Finite element model (: 
model dimensions in inches). 



30 33.5 
showing 



strength (shear strength) of the rock 
(coal) encompassing the tendon. 

The computer analysis provided both 
center-point and nodal-point data for 
each element. Postprocessing of the 
original computer values revealed nearly 
linear dissipations of the applied loads 
through the length of the bolt. However, 
linear load dissipations did not corre- 
late with the actual field measurements. 
To improve correlation and similitude, 
and to ensure compatibility with the 
original field measurements, the finite 
element model was slightly modified (10). 
The required modification included the 
addition of bolt displacements. The ap- 
proximate bolt displacements, which were 
measured with a dial gauge during the 
field analysis, were added to the first 
one-third of the bolt length measured 
from the collar of the hole. The re- 
sults, after this modification, compared 
favorably to the field data. 

A typical representation of percent 
load versus distance is shown in figure 
14. The plot shows how rapidly the 
load dissipates when applied to a fully 
grouted bolt. At a depth of 20 in, only 
50% of the initial load is realized in 



45 



Y = 62.98 exp 
2 




-0.I3I3(X) 



5 20 25 30 35 
DISTANCE, in 



40 45 50 



FIGURE 14. - Typical plot of percent of ap- 
plied load versus boltdistance (r = coefficient 
of correlation). 



11 



the bolt. The load dissipations were 
similar for all of the tested loads. It 
was shown in the field tests that the 
transfer of the load in the resin-grouted 
bolt system took place almost totally 
within the bolt at the load levels 
tested. 

When load is applied to the bolt, the 
bolt stretches in the longitudinal direc- 
tion and contracts in the traverse di- 
rection due to Poisson's effect. The 
contraction of the bolt will cause the 
bond to be broken between the bolt and 
the resin grout. If the loads are high 
enough, the contraction will cause the 
bolt to separate from the resin grout at 
the end. The length of this separation 
will increase with increasing loads. The 
amount of contraction in the anchor de- 
cays rapidly as the distance from the 
bolt head increases. 



At a constant stress on the head of the 
bolt and at varying coal compressive 
strengths of 1,000, 1,500, 2,000, 3,500, 
5,000, 7,500, and 10,000 psi, no signifi- 
cant changes in the stress distributions 
were observed. This was the result of 
the high values for the Young's modulus 
for the bolt as opposed to the low value 
for the coal (60:1). Safety factors were 
calculated, based on Mohr-Coloumb failure 
criteria, to determine the effect varying 
compressive strength values would have on 
the host rock. Results indicated that 
when the compressive strength is 2,000 
psi or less, the safety factors approach 
critical levels. These safety factors 
indicate that the material 4 in up from 
the hole collar is in tension, which may 
cause failure. 



CONCLUSIONS 



All of the roof bolting systems tested 
met testing criteria required for thick 
top coal roofs with the exception of the 
type of expansion anchor bolts tested. 

The finite element model and the 
strain-gauge bolts provided insights on 
load dissipation and anchorage behavior. 
It is recognized, however, that the re- 
sults of the finite element study were 
dependent on the accurate determination 
of the host rock properties. Tests on 



fully instrumented pull-tested bolts in 
the same host rock as the model provided 
a validation of the numerical model. 

The anchor load-transfer mechanism must 
be assessed to provide a complete pic- 
ture of the stresses and strains around a 
fixed anchor. 

It is believed that thick coal roofs 
can be fully supported in the bottom por- 
tion of the coal roof without anchoring 
in the immediate host rock. 



REFERENCES 



1. Karabin, G. J., and W. J. Debevec. 
Comparative Evaluation of Conventional 
and Resin Bolting Systems. MESA (now 
MSHA) IR 1033, 1976, 22 pp. 

2. Karabin, G. J., and M. T. Hoch. 
An Operational Analysis of Point Resin- 
Anchored Bolting Systems, MSHA IR 1100, 
1979, 14 pp. 

3. . Contemporary Roof Support 

Systems Provide a Diversified Approach 
to Modern Ground Control. Paper in Pro- 
ceedings, Annual Institute on Coal Mine 
Health and Safety Meeting. VA Polytech. 
Inst., 1980, pp. 249-268. 

4. Kwitowski, A. J., and L. V. Wade. 
Reinforcement Mechanisms of Untensioned 



Full-Column Resin Bolts. BuMines RI 
8439, 1980, 27 pp. 

5. Brask, C. G., and H. Hamrin. New 
Type of Rockbolt Simplifies Rock Rein- 
forcement Procedures. Atlas Copco, Swe- 
den, 1983, pp. 1-17. 

6. Nitzche, R. N. , and C. J. Haas. 
Installation Induced Stresses for Grouted 
Roof Bolts. Int. J. Rock Mech. , Min. 
Sci. , and Geomech. , v. 13, No. 1, 1976, 
pp. 17-24. 

7. International Society for Rock 
Mechanics (Commission on Standardization 
of Laboratory and Field Test, Committee 
on Field Tests). Suggested Methods for 



12 



Rockbolt Testing. No. 2, March 1974, 
pp. 1-16. 

8. Bathe, K. A Finite Element Program 
for Automatic Dynamic Incremental Non- 
linear Analysis (ADINA). Acoustics and 
Vibrations Lab., Mech. Eng. Dep., MIT, 
Cambridge, MA. Rep. 82448-1 Sept. 1975, 
p. 360. 

9. Yap, L. D. , and A. A. Rodger. A 
Study of the Behavior of Vertical Rock 
Anchors Using the Finite Element Method. 



Int. J. Rock Mech., Min. Sci., and Geo- 
mech. , v. 21, No. 2, 1984, pp. 47-61. 

10. Serbousek, M. 0., and S. P. Sig- 
ner. Load Transfer Mechanics in Fully 
Grouted Roof Bolts. Pres. at 4th Confer- 
ence on Ground Control in Mining, Mor- 
gantown, WV, July 22-24, 1985, 11 pp.; 
available from M. 0. Serbousek, U.S. Bu- 
reau of Mines, Spokane Research Center, 
Spokane, WA. 



13 



APPENDIX. — TORQUE-TENSION RATIO TESTS 



Torque-tension ratio tests determine 
the amount of torque necessary for the 
proper installation of mechanical expan- 
sion anchor and combination bolts. The 
test procedure requires that a calibrated 
hydraulic U-cell be installed between the 
bearing plate and the mine roof. The 
bolt is then tightened by turning its 
head with a torque wrench. The values 
of the torque wrench are recorded in con- 
junction with the value on the U-cell. 
When the value on the U-cell drops, the 
maximum anchorage capacity of the bolt 
system has been attained. Bolts should 
then be tensioned to 50% to 70% of the 
yield strength of the bolt or 50% to 
70% of the anchorage capacity of the 
material, whichever is less. The recom- 
mended torque-tension ratio is 50 lb ten- 
sion per foot pound of torque for bolts 
without hardened washers or 60 lb tension 
per foot pound of torque with hardened 
washers. 

An example follows: Find the torque 
range for a 5/8-in grade 55 bolt with 



no hardened washer when the rock anchor- 
age capacity is 15,000 lb. The yield 
strength for a grade 55 bolt is 12,400 
lb, which is less than the anchorage 
capacity. Therefore, the yield strength 
is used for design purposes, as shown 
below. 

Lower-limit calculation 

12,400 lb 
x 50% 
6,200 lb 

6,200 lb ... ... .. . 

50 lb/lbf = 124 lbf * ft t0rqUe 

Upper-limit calculation 

12,400 lb 
x 70% 



8,680 lb 



8,680 lb .,, ... .. . 

50 Ibf/ft = 174 lbf ft t0rque 



ftU.S. CPO: 1985-605-017/20,135 



IN T.-BU.O F MIN ES,PGH.,P A. 28(66 



U.S. Department of the Interior 
Bureau of Mines— Prod, and Distr. 
Cochrans Mill Road 
P.O. Box 18070 
Pittsburgh. Pa. 15236 



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